1. Field of the Invention
The present invention relates to a high-voltage transformer and a discharge lamp driving apparatus which are used, for example, in a lighting circuit of a discharge lamp for backlight in a liquid crystal display panel and, in particular, to a high-voltage transformer and a discharge lamp driving apparatus, used in a DC/AC inverter circuit, for simultaneously lighting a plurality of discharge lamps.
2. Description of the Prior Art
It has conventionally been known to discharge/light a plurality of cold cathode fluorescent lamps (hereinafter referred to as CCFLs) simultaneously as backlight for various liquid crystal display panels used in notebook PCs, for example. Using a plurality of CCFLs as such can respond to demands for higher luminance and uniform illumination in liquid crystal display panels.
Known as a typical circuit for lighting this kind of CCFL is an inverter circuit which converts a DC voltage of about 12 V into a high-frequency voltage of about 2,000 V or higher at 60 kHz by using a high-voltage transformer, so as to start discharging. After the discharging is started, the inverter circuit regulates the high-frequency voltage so as to lower it to a voltage of about 800 V which is required for keeping the discharge of CCFL.
As high-voltage transformers (inverter transformers) used in such an inverter circuit, those with a small size have been in use in view of the demand for making liquid crystal display panels thinner. Since the high-voltage transformers are necessary by the number of CCFLs in a single liquid crystal display, there is an urgent need for establishing a technique for further saving their space and manufacturing cost. Known as an example responding to such a need is the discharge lamp driving circuit shown in FIG. 12.
This discharge lamp driving circuit is configured such that a DC input voltage is fed to the primary side of a high-voltage transformer 610 by way of a known Royer oscillation circuit 600, so as to generate a high voltage of about 2,000 V or higher on the secondary side of the high-voltage transformer 610 at the time when discharge lamps start lighting, whereas the high voltage on the secondary side is applied to cold cathode fluorescent lamps CCFL1, CCFL2 by way of ballast capacitors Cb1, Cb2, respectively. Connecting the ballast capacitors Cb1, Cb2 to the CCFL1, CCFL2, respectively, in series can eliminate fluctuations in the starter voltage of each lamp, whereby a plurality of CCFLs can be lit by a single transformer while suppressing fluctuations in the discharging operation of each CCFL.
However, a voltage of (1,600 to 2,000 V between both ends of a CCFL) 2 to 2.5 times that at the time of normal lighting (800 V between both ends) is necessary at the time when the CCFL starts lighting, and a voltage of about 400 V or higher is divisionally applied between both ends of a ballast capacitor Cb connected thereto, whereby a high voltage of at least about 2,000 V is continuously outputted from the secondary side of the transformer when the CCFL starts lighting and keeps normally lighting.
Continuously outputting such a high voltage lowers the reliability of the transformer, thus making it difficult to secure safety against the isolation voltage between turns of the secondary coil in the transformer and the like.
The secondary voltage may be varied between when the CCFL starts lighting and lights normally, so that the voltage is lowered at the time of normal lighting. However, the high-voltage transformer 610 has no function to regulate its voltage. Though the circuit part for driving the high-voltage transformer 610 has a PWM control function in general, this is usually a voltage control function for keeping the lamp lighting at the time of normal lighting, whereby it is essentially difficult to switch a starter voltage of about 2,000 V or higher to a normal lighting voltage of about 800 V.
Therefore, when employing a technique for switching the secondary voltage between the initial lighting time and the normal lighting time, a configuration basically different from conventional ones is required to be developed.